1. Field of the Invention
The present invention relates generally to an fluid pressure control arrangement for a vehicular automatic transmission. Specifically, the present invention relates to an arrangement which controls transmission line fluid pressure to maintain sufficient engagement capacity of frictionally engageable elements of the transmission during shifting.
2. Description of the Prior Art
Conventionally, fluid pressure control in an automatic transmission is known in the art. In such conventional automatic transmissions, for example, such as disclosed in U.S. Pat. No. 4,730,521, it is well known to adjust fluid pressure during shifting according to a throttle valve opening angle.
When a transmission is new, the frictional coefficients of frictionally engaged transmission components is substantially high, however, after an initial `breaking in` period, as the frictionally engageable elements are subject to use, this frictional coefficient becomes reduced. The line pressure in the transmission must insure sufficient pressure to maintain suitable engagement capacity of the frictionally engageable elements after this `breaking in` period to assure normal transmission operation. This means however, that when the transmission is new and the frictional coefficient of frictionally engageable elements is high, a higher than necessary line pressure is present in the transmission causing an undesirable `shift shock` to occur during shifting in a new transmission. However, in conventional automatic transmissions, the line pressure is always adjusted to the same degree according to the same throttle valve angle even when the transmission is new and therefore such an undesirable `shift shock` feeling cannot be prevented.